1. Field of the Invention
The present invention relates to a start-up circuit in a power converter to monitor line-in voltage to switch on or off the power converter.
2. Description of the Related Arts
A switching power converter generally includes a controller integrated chip (IC) that controls a switch for coupling or decoupling a load to or from a power source. Such controller IC includes, among other components, a start-up circuit. The start-up circuit monitors line-in voltage provided at the input of the power converter circuit and turns on components of the controller IC if the line-in voltage is higher than a start-up voltage. Conversely, if the line-in voltage drops below a threshold voltage (i.e., Undervoltage Lockout voltage or UVLO voltage), the start-up circuit shuts down the components of the controller IC. Even in no-load conditions, the start-up circuit remains operational and consumes power.
One of the major design criteria for the start-up circuit is low power consumption. As more energy efficient power converters become required, power consumption has become one of the major design criteria in designing power converters. Some AC-DC power converters, for example, are required to consume less than 30 mW under no-load conditions. In order to satisfy such strict requirement, the start-up circuit needs to consume as low power as possible. Another important design consideration for the start-up circuit is space occupied by the start-up circuit. To reduce the size of the controller IC and include more functionality in the power converters, the size of the start-up circuit also needs to be reduced.
FIG. 1 is a conventional start-up circuit 100 in a controller IC. Start-up circuit 100 generates an enable signal (EN) to operate components of a power converter, including main bandgap circuit 150. Supply voltage is provided across Vcc node and GND node connected to start-up circuit 100 and main bandgap circuit 150. Start-up circuit 100 of FIG. 1 includes a Low-Dropout (LDO) regulator 110, a bandgap block 114, a string of sensing resistors (R1 through R3), a multiplexer 118, a comparator 120, and a current generator 124. LDO 110 provides voltage signal Vd to the bandgap block 114 via node N4 and to the comparator 120 via node N6. The bandgap block 114 is connected between node N4 and ground (GND) to generate reference voltage signal VB1. Reference voltage signal VB1 is fed to the inverted input of the comparator 120 via node N2 and to an input of LDO 130 in the main bandgap circuit 150.
Comparator 120 turns EN signal on or off based on reference voltage signal VB1 from bandgap block 114. Specifically, the comparator 120 turns on EN signal when supply voltage Vcc to the start-up circuit 100 increases above a start-up voltage. Conversely, the comparator 120 turns off EN signal when supply voltage Vcc drops below a UVLO voltage.
Multiplexer 118 selectively connects the non-inverted input of the comparator 120 to node N3 or node N5 based on EN signal from the comparator 120. When EN signal is turned off, the multiplexer 118 connects node N5 to the non-inverted input of the comparator 120. Hence, the voltage across resistors R1 and R2 is received at the non-inverted input of the comparator 120. In contrast, if EN signal is turned on, multiplexer 118 connects node N3 to the non-inverted input of the comparator 120. In this way, the start-up voltage at which the comparator 120 turns on EN signal and the UVLO voltage at which the comparator 120 turns off EN signal can be set differently. Current generator 124 is turned on by EN signal to provide operating current to LDO 130 and bandgap block 134.
Main bandgap circuit 150 includes LDO 130 and bandgap block 134. LDO 130 receives voltage signal VB1 from the bandgap block 114 and EN signal from comparator 120. Voltage signal VB1 functions as a reference voltage signal for LDO 130. LDO 130 also receives current IL from the current generator 124. LDO 130 generates regulated voltage Vf2 that is fed to the input of bandgap block 134. Bandgap block 134 receives current IB and voltage Vf2, and generates reference voltage VBG that is more reliable and less susceptible to process, voltage and temperature (PVT) variations compared to voltage signal VB1. The reference voltage VBG is used by various components of the controller IC as a reference voltage.
Startup-circuit 100 of FIG. 1, however, consumes a large amount of power (e.g., about 10 μA at start-up voltage of 10V) and takes up a large area within the controller IC partly due to the presence of LDO 110 and bandgap block 114.